Allow generic Redis providers via env var

Instead of hardcoding every provider's environment variable, Sidekiq now
supports both REDIS_PROVIDER and REDIS_URL env vars.

Let's say you run on Heroku and you use openredis. You can now set
REDIS_PROVIDER=OPENREDIS_URL and Sidekiq will use the openredis uri.

If you switch to another provider, all you need to do is reset
REDIS_PROVIDER. An example of switching to RedisGreen:
REDIS_PROVIDER=REDISGREEN_URL. Easy peasy!

Sidekiq also supports using your own url with REDIS_URL. While you can
still pass in the URL to Sidekiq::RedisConnection.create, it's also now
possible to set it with just an env var.

Closes #443

Changes.md

@@ -1,3 +1,8 @@
+HEAD
+-----------
+
+- Allow generic Redis provider as environment variable. [#443]
+
 2.3.3
 -----------
 

lib/sidekiq/redis_connection.rb

@@ -5,7 +5,7 @@ require 'redis/namespace'
 module Sidekiq
   class RedisConnection
     def self.create(options={})
-      url = options[:url] || ENV['REDISTOGO_URL'] || 'redis://localhost:6379/0'
+      url = options[:url] || determine_redis_provider || 'redis://localhost:6379/0'
       driver = options[:driver] || 'ruby'
       # need a connection for Fetcher and Retry
       size = options[:size] || (Sidekiq.server? ? (Sidekiq.options[:concurrency] + 2) : 5)
@@ -24,5 +24,15 @@ module Sidekiq
       end
     end
     private_class_method :build_client
+
+    def self.determine_redis_provider
+      provider = if ENV.has_key? 'REDISTOGO_URL'
+        'REDISTOGO_URL'
+      else
+        ENV['REDIS_PROVIDER'] || 'REDIS_URL'
+      end
+      ENV[provider]
+    end
+    private_class_method :determine_redis_provider
   end
 end